[llvm] [NFC][CodeGen] Add helper function to check SubReg validity (PR #181489)

[Rahul Joshi via llvm-commits]

jurahul wrote:

I added another overload to check if RC/subReg combination is valid for use with DRC. Also, as a part of this I eliminated he use of `getLargestLegalSuperClass` as that does not seem required for this validation.

https://github.com/llvm/llvm-project/pull/181489